Hydrocarbon resources such as petroleum or natural gas have been mined and produced through wells having a porous and permeable subterranean formation (wells or gas wells; also collectively called wells). Well depths have progressively increased in step with increases in energy consumption. There are records of drilling to depths exceeding 9,000 m around the world, and there are wells over 6,000 m deep in Japan. In wells with ongoing mining, in order to continuously mine hydrocarbon resources efficiently from a subterranean formation whose permeability has diminished over time or a subterranean formation which originally has insufficient permeability, the productive layer is stimulated, and acid treatment or crushing methods are known as stimulation methods (Patent Document 1). Acid treatment is a method of increasing the permeability of the productive layer by infusing a mixture of a strong acid such as hydrochloric acid or hydrogen fluoride into the productive layer and dissolving reactive components of the bedrock (carbonates, clay minerals, silicates, or the like), but various problems associated with the use of strong acids have been indicated, and increases in cost, various countermeasures, have also been indicated. Therefore, attention has been focused on a method of forming fractures in the productive layer by utilizing fluid pressure (also called a “fracturing method” or a “hydraulic fracturing method”).
Hydraulic fracturing is a method of generating fractures in the productive layer by means of fluid pressure such as water pressure (also simply called “water pressure” hereafter) and is typically a productive layer stimulation method of well drilling a vertical hole, bending the vertical hole, well drilling a horizontal hole in the stratum several thousand meters underground, feeding a fracturing fluid into the well holes (referring to holes provided to form wells; also called “downholes”) under high pressure, producing fractures in the productive layer at a high depth underground (layer for producing hydrocarbon resources such as petroleum or natural gas) with water pressure, and extracting the hydrocarbon resources through the fractures. The efficacy of hydraulic fracturing has also been the focus of attention in the development of non-conventional resources such as shale oil (oil matured in shale) or shale gas.
Fractures formed by fluid pressure such as water pressure is immediately closed by formation pressure once the water pressure is eliminated. In order to prevent the closure of fractures, a proppant is added to the fracturing fluid (that is, a well treatment fluid used for fracturing) and fed into the well hole so as to place the proppant in the fractures. An inorganic or organic material is used as the proppant contained in the fracturing fluid, but silica, alumina, or other inorganic particles are conventionally used since the closure of fractures can be prevented in high-temperature, high-pressure environments deep underground over as long a period as possible, and grains of sand—for example, 20/40 mesh sand or the like—are widely used.
Various types of water-based, oil-based, and emulsion-based well treatment fluid are used as the fracturing fluid. The well treatment fluid must have a functional capable of carrying the proppant to a location where fractures are to be produced in the well hole, so the well treatment fluid ordinarily must have a prescribed viscosity as well as good proppant dispersibility, and there is a demand for the ease of after-treatment and a small environmental burden. In addition, the fracturing fluid may also contain a channelant for the purpose of forming channels through which shale oil, shale gas, or the like can pass between the proppants. Therefore, various additives such as channelants, gelling agents, scale inhibitors, acids for dissolving rock or the like, and friction reducers are used in the well treatment fluid in addition to proppants.
In order to generate fractures in productive layer deep underground (the layer for producing hydrocarbon resources including petroleum such as shale oil or natural gas such as shale gas) with water pressure using a fracturing fluid, the following method is ordinarily employed. Specifically, for a well hole (downhole) bored into the stratum several thousand meters underground, prescribed sections are partially isolated while plugging sequentially from the end of the well hole, and fracturing is performed to generate fractures in the productive layer by infusing a fracturing fluid at high pressure into the isolated sections. Next, a prescribed section (ordinarily in front of the preceding section—that is, a section on the surface side) is isolated and fractured. This process is performed repeatedly thereafter until the required plugging and fracturing are complete.
The stimulation of the productive layer by means of secondary fracturing is performed not only for the drilling of a new well, but also for a desired section of a well hole that has already been formed. In this case as well, an operation of isolating the well hole and performing fracturing may be similarly performed. In addition, in order to finish the well, the well hole may be isolated so as to isolate the fluid from the lower part, and the isolation may be removed after the upper part is finished.
There are various known methods of isolating a well hole, performing fracturing, or the like. For example, plugs capable of isolating or fixing a well hole (also called “frac plugs”, “bridge plugs”, “packers”, or the like) are disclosed in Patent Documents 2 and 3.
A downhole plug for well drilling (also simply called a “plug” hereafter) is disclosed in Patent Document 2. Specifically, Patent Document 2 discloses a plug provided with a mandrel (main body) having a hollow part in the axial direction and a ring or annular member, a first conical member and slip, a malleable element formed from an elastomer, a rubber, or the like, a second conical member and slip, and an anti-rotation feature along the axial direction on the outer peripheral surface existing in the orthogonal to the axial direction of the mandrel. The sealing of a well hole with this downhole plug for well drilling is as follows. Specifically, by moving the mandrel in the axial direction thereof, the slips make contact with the inclined surface of the conical member and advance along the conical members as the gap between the ring or annular member and the anti-rotation feature is reduced. As a result, the slips expand radially outward and make contact with the inside wall of the well hole so as to be fixed to the well hole, and the malleable element expands in diameter, deforms, and makes contact with the inside wall of the well hole so as to seal the well hole. The mandrel has a hollow part in the axial direction, and the well hole can be sealed by setting a ball or the like in the hollow part. A wide range of materials such as metal materials (aluminum, steel, stainless steel, and the like) fibers, wood, composite materials, and plastics are given as examples of materials for forming the plug. It is described that the material is preferably a composite material containing a reinforcing material such as carbon fibers and particularly a polymer composite material such as an epoxy resin or phenol resin, and that the mandrel is formed from aluminum or a composite material. On the other hand, it is described that in addition to the materials described above, materials which decompose due to temperature, pressure, pH (acid, base), or the like can be used as the ball or the like.
Downhole plugs for well drilling are successively placed in the well until the well is complete, but they may need to be removed at the stage when the production of petroleum such as shale oil or natural gas such as shale gas (also collectively called “petroleum or natural gas” or “petroleum and/or natural gas” hereafter) or the like is begun. Plugs are not ordinarily designed to be retrievable by removing the isolation after use and are removed as a result of being destroyed or fragmented by crushing, well drilling, or another method, but crushing, well drilling, or the like required a large amount of time and money. In addition, there are also plugs specially designed so as to be retrievable after use (retrievable plugs), but since the plugs are placed deep underground, a large amount of time and money were required to recover all of the plugs.
Patent Document 3 discloses a disposable downhole tool (meaning a downhole tool or the like) containing a biodegradable material which degrades when exposed to the environment inside a well, and a member thereof, and degradable polymers including aliphatic polyesters such as polylactic acid are disclosed as biodegradable materials. Further, Patent Document 3 discloses a combination of a tubular body element having a flow bore in the axial direction and a combination of a packer element assembly comprising an upper sealing element, a central sealing element, and a lower sealing element and a slip and a mechanical slip body along the axial direction on the outer peripheral surface existing in the orthogonal to the axial direction of the tubular body element. In addition, it is disclosed that the flow of a fluid is permitted in only one direction by setting a ball in the flow bore of the tubular body element. However, there is no disclosure in Patent Document 3 as to whether a material containing a biodegradable material is used for either the downhole tool or the member thereof.
In response to increasing demands for the procurement of energy resources, environmental protection, and the like, and as the mining of non-conventional resources expands, in particular, there has been a demand for a plug for well drilling process which enables the reliable isolating and fracturing of a well hole and is capable of reducing the cost of well drilling and shortening the process by facilitating the removal of the plug for well drilling process and the procurement of a flow path.